1. Field of the Invention
The present invention relates to an automatic equalizer in a modulator/demodulator for performing data transmission by utilizing a line.
2. Description of the Prior Art
When digital signal data is transmitted through a general public line (analog line), a modulator/demodulator (modem) for converting a digital signal into a desired analog signal, and vice versa is required. The modulator/demodulator normally includes an adaptive type automatic equalizer for correcting line characteristics. Prior to data transmission, the modulator/demodulator transmits a training signal to confirm, e.g., transmission characteristics of the line, and adjusts equalization characteristics of the equalizer, thus determining correction specifications of transmission characteristics defined by, e.g., the line characteristics. Thereafter, equalization processing according to the correction specifications is executed, so that the equalizer characteristics are changed to follow a slow change in line characteristics over time during data transmission. Such equalization processing is normally called automatic equalization or adaptive equalization.
FIG. 5 shows an arrangement of a conventional adaptive type automatic equalizer. FIG. 5 exemplifies a case wherein the equalizer comprises a transversal filter.
In FIG. 5, reference numeral 200 denotes a demodulator for converting a modulated signal received from a line into a complex baseband signal. Subsequently, the complex baseband signal from the modulator 200 is supplied to a complex conjugator 203, and is then converted into a complex conjugate baseband signal. The complex conjugate baseband signal is sequentially input to a delay circuit 204 constituting a transversal filter. The delay circuit 204 delays a complex by a predetermined period of time.
In FIG. 5, [(C.sub.-N).sup.r+1, . . . , (C.sub.0).sup.r+1, . . . , (C.sub.N).sup.r+1 ] represent tap gains to be multiplied with outputs from delay circuit circuits located immediately thereabove by multipliers 205. Reference numeral 207 denotes an adder for calculating a total sum of products of the delayed complex conjugate baseband signal, and the tap gains.
An equalizer output signal Y.sub.k of a conventional equalizer constituted by the complex conjugator 203, the delay circuit 204, the multipliers 205, and the adder 207 is given by: ##EQU1## where * represents complex conjugate, and (R.sub.k-i) * is the ith complex conjugate baseband signal to the equalizer. (C.sub.i).sup.r+1 is the value calculated in an (r+1)th calculation of an ith tap gain of the transversal filter.
The equalizer output Y.sub.k given by equation (1) is input to a discriminator 210. The discriminator 210 has a plurality of reference signal points. The discriminator 210 selects a point having the shortest distance to Y.sub.k as a discrimination point of Y.sub.k from these reference signal points, and outputs it as a point a. Note that a difference E.sub.k =Y.sub.k -a.sub.k between the equalizer output Y.sub.k and the discrimination point a.sub.k corresponds to an equalization error at time k.
In this manner, the conventional adaptive type automatic equalizer is a circuit for sequentially correcting tap gains [C.sub.-N . . . C.sub.0 . . . C.sub.N ] so as to minimize the mean square error of E. Each tap gain of the conventional adaptive type automatic equalizer is updated according to the following equation: EQU (C1).sup.r+1 =(C1).sup.r -.alpha..multidot.(R.sub.k-1)*.multidot.E.sub.k ( 2)
where .alpha. is a constant that influences an equalization rate of the adaptive type automatic equalizer, and is generally called a convergence coefficient.
A circuit for updating the tap gains given by equation (2) is constituted by the delay circuits 204 located immediately below the tap gains (FIG. 5), the multipliers 205, adders 206, and a switch 209. Note that the switch 209 outputs the convergence coefficient .alpha..
When data transmission is performed using the modulator/demodulator provided with the above-mentioned adaptive type automatic equalizer, if a line is instantaneously disconnected or suffers from an abrupt level drift, equalization processing is assumed to be adaptively performed according to the tap gain correction algorithm given by equation (2). In this case, the equalization error E.sub.k assumes a very large value, and the tap gains unexpectedly change largely.
As a result, equalization performance of the adaptive type automatic equalizer is immediately impaired. Thereafter, discrimination errors successively occur, or when the degree of the line trouble is serious, the equalizer diverges, and its equalization performance may never be recovered.
A conventional countermeasure against impaired equalization performance or divergence of the adaptive type automatic equalizer due to the above-mentioned line trouble will be explained below.
In FIG. 5, a modulated signal received from the line is subjected to level adjustment by AGC (automatic gain control) having a time constant, which is large enough not to be influenced by the amplitude drift inherent to the modulated signal, before the signal is input to the demodulator 200. The level-adjusted modulated signal received from the line is demodulated into a complex baseband signal by the demodulator 200. For this reason, when the complex baseband signal from the demodulator 200 is input to a detector 201 that detects the amount of change in the level of the signal (hereinafter the "level change amount detector 201"), only the amount of change in the level corresponding to an instantaneous disconnection or an abrupt level drift, which cannot be level-adjusted by the AGC prior to the demodulator 200, is detected. The detected amount of change in the level is compared with a comparison value .beta. selected in advance by a comparator 202. If the amount of change in the level is larger than the value .beta., the switch 209 outputs a convergence coefficient "0"; otherwise, the switch 209 outputs the convergence coefficient .alpha.. Note that the comparison value .beta. is a constant determined within a range of the amount of change in the level wherein no discrimination error occurs.
The above description can be rephrased as follows.
When the line is instantaneously disconnected or suffers from an abrupt level drift, and the comparator 202 determines that it is not preferable to correct the tap gains of the adaptive type automatic equalizer due to the line abnormality, the switch 209 selects the convergence coefficient .alpha.=0. For this reason, equation (2) for updating the tap gains is rewritten as (C.sub.1).sup.r+1 =(C.sub.1).sup.r, thereby freezing the tap gains On the other hand, when the line abnormality is recovered, the switch 209 selects the convergence coefficient .alpha., and the updating operation of the tap gains based on equation (2) is restarted, so that the adaptive type automatic equalizer can start sequential line equalization.
However, in the above-mentioned prior art, when a line abnormality such as an instantaneous disconnection or an abrupt level drift is detected, since the updating operation of the tap gains of the adaptive type automatic equalizer is frozen, line characteristics may drift over time, or synchronization of timing phases or carrier phases may be considerably shifted when the line abnormality is detected for a long period of time.
Under such circumstances, even if the line abnormality is recovered, and the updating operation of the tap gains is restarted, the equalization error is large, and as a result, discrimination errors frequently occur, resulting in divergence of the adaptive type automatic equalizer.